1. Field of the Invention
The present invention generally relates to multiplexed analog component (MAC) television systems and, more particularly, to an improved interface between a subscriber and an integrated receiver-decoder (IRD) in a MAC television system.
2. Description of the Relevant Art
For the purposes of the following discussion and this invention, the term "subscriber" means one who is receiving a television service. The "subscriber" could thus be an individual consumer with a decoder in his own home, or could be a system operator such as a local cable TV operator, or a small network operator such as a hotel/motel operator with a central decoder for all televisions in the hotel or motel. In addition, the "subscriber" could be an industrial user, as described in U.S. Pat. No. 4,866,770 assigned to the same assignee as the present application and incorporated herein by reference.
For the purposes of this invention, a network is defined as a program source (such as a pay television provider), an encoder (sometimes called a "head end"), a transmission means (satellite, cable, radio wave, etc.) and a series of decoders used by the subscribers. A system is defined as a program source, an encoder, a transmission means, and a single receiving decoder. The system model is used to described how an individual decoder in a network interacts with the encoder.
A MAC color television signal is illustrated in FIG. 1, which is an amplitude-vs.-time diagram of a single video line of 63.56 microseconds duration. The first 10.9 microseconds is the horizontal blanking interval (HBI) 22, in which no picture information is transmitted. Following HBI 22 are chrominance signal 24 and luminance signal 26, either of which may be time-compressed. Between chrominance signal 24 and luminance signal 26 is a 0.28 microsecond guard band 28, to assist in preventing interference between the two signals.
The MAC color television signal of FIG. 1 is obtained by generating conventional luminance and chrominance signals (as would be done to obtain a conventional NTSC or other composite color television signal) and then sampling and storing them separately. Luminance is sampled at a luminance sampling frequency and stored in a luminance store, while chrominance is sampled at a chrominance sampling frequency and stored in a chrominance store. The luminance or chrominance samples may then be compressed in time by writing them into the store at their individual sampling frequency and reading them from the store at a higher frequency. A multiplexer selects either the luminance store or the chrominance store, at the appropriate time during the active video line, for reading, thus creating the MAC signal of FIG. 1. Audio samples may be transmitted during the HBI; these are multiplexed (and may be compressed) in the same manner as the video samples. The single rate at which all samples occur in the MAC signal is called the MAC sampling frequency.
FIG. 2 shows a prior art conditional-access system for satellite transmission. In encoder 101, the source program information 102 which comprises video signals, audio signals, and data is scrambled in program scrambler 103 using a key from key memory 104. The scrambling techniques used may be any such techniques which are well known in the art. The key can be a signal or code number used in the scrambling process which is also required to "unlock" or descramble the program in program descrambler 108 in decoder 106. In practice, one key can be used (single layer encryption) or more than one key (not shown). The key is usually changed with time (i.e.--monthly) to discourage piracy. The scrambled programs and the key are transmitted through satellite link 105, and received by conditional-access decoder 106. Decoder 106 recovers the key from the received signal, stores it in key memory 107 and applies it to program descrambler 108 which descrambles the scrambled program received over satellite link 105, and outputs unscrambled program 109.
FIG. 3 shows the overall transmission format of a MAC system. As is conventional in television, 30 "frames" each comprising a still image are transmitted per second as indicated. Each frame includes two "fields," as also shown. In a preferred embodiment of the invention, the video encoding scheme employed is that referred to generally as "B-MAC." This is an acronym for type B format, Multiplexed Analog Component system. "Type B" refers to the fact that data is carried integral to the video signal. See generally Lowry, "B-MAC: An Optimum Format for Satellite Television Transmission," SMPTE Journal, November 1984, pp. 1034-1043, incorporated herein by reference, which discusses in detail the B-MAC format and explains why it was chosen over various competing systems.
The vertical blanking interval (VBI) of each field contains certain "system data" necessary for operation of a subscription television system as well as addressed packets and teletext lines used to carry data needed for the operation of individual decoders and for transmission of messages to individual subscribers. Preferably, the vertical blanking intervals of 16 total fields are used for complete transmission of all system data required, which includes an encryption key which is changed every 16 fields, that is, on the order of three times per second. As also shown in FIG. 3, each line also includes a horizontal blanking interval (HBI). During the HBI are transmitted six channels of high quality digitally-encoded audio information, with error correction, such that the decoder can also be used to supply a high quality audio signal. This can be used to provide the audio component of the corresponding video signal (or several versions thereof, in different languages) or an additional audio signal, such that subscription audio is also made available according to the system of the invention.
FIG. 4 shows the format of the horizontal blanking interval (HBI). The HBI perferably consists of 78 total bits of pulse amplitude modulated data. The HBI is interposed between vertical blanking interval or video information from a previous line and that of the present line. A typical horizontal blanking interval as shown begins with a two-bit guard band 30, followed by 45 bits of audio and utility data 32, a second two-bit guard band 34, twenty bits of color burst information 36, a further guard band 38, six more bits of data 40 and a final guard band 42, after which the VBI or the video signal of the particular frame commences. The position of the color burst 30 within the HBI varies, to provide signal scrambling. Descrambling involves the use of a repetitively-transmitted key.
FIG. 5 shows some additional details of the horizontal blanking interval data 32 and 40 shown in FIG. 4. In the example shown, fifty-one total bits of data are provided in each line of the HBI, and each bit is pulse amplitude modulated encoded, such that each bit period includes transmission of two bits. One bit can be referred to as sign and the other as magnitude as indicated on FIG. 5. As shown, the first seventy-eight bits are digital audio. Thus each frame provides a thirteen-bit digital representation of a sample of each of six audio channels. High quality transmission of audio frequencies up to approximately 15 kHz is thus provided. Following the audio information are six bits of stepsize and bandwidth information. The stepsize bits indicate the size of the steps numbered by the thirteen bits of information preceding, and the bandwidth information relates to the amount of the amount of emphases or de-emphasis of the signal employed. Alternate fields carry the stepsize and bandwidth data. Both these terms are used as conventional in the Dolby delta modulation scheme, which is employed in the preferred embodiment of this invention for transmission of the audio. Following are twelve bits of error correction code (ECC) for correction of the audio, indicated at 48. Four utility bits follow at 50, and the last bit 52 of the data are a parity check bits for checking the parity of the error correction bits 48.
FIG. 6 shows the arrangement of the lines which make up the vertical blanking interval (VBI). The VBI includes 16 lines in the 525-line NTSC version of this invention. A slightly different number of lines are used in the 625-line PAL. The functions of the lines and their arrangement in other respects are identical.
As indicated, the vertical blanking interval is 377 bits wide. These bits are pulse amplitude modulated FSK scheme used in the HBI as discussed above. Lines 1, 2 and 3 includes the transmission of clock recovery, synchronization and system service, as indicated in FIG. 6.
For the purposes of the present invention, the significant data contained in line 3 is a system key which is updated every sixteen fields, that is, which changes with each complete system data transmission as indicated above in connection with FIG. 3. The system key is common to all decoders. The system key is contained in the service data of line 3, and is used for decryption of video program material, audio and teletext.
Lines 4-8 of the VBI include the addressed packets, as indicated by reference numeral 62. As noted, these each contain an address which is then followed by data, concluding with error correction coding (ECC). The addresses are those of the individual decoders. The addresses in the address packets are transmitted in clear text, such that they can be received without decryption by the receiver. The remainder of the message is encrypted. In this way, addressed packet data, which is, very significant to the proper functioning of the system because one of the addressed packets includes one of the decrypting ciphers needed, is provided with a high degree of security. Addressed packets addressed to differing decoders may be transmitted in a single field.
As indicated at 64, lines 9-13 of the VBI are used to transmit teletext. The first part of each teletext line is a teletext identification which indicates that the line in fact is teletext. As shown, two types of teletext lines are used. Teletext headers include a relatively larger number of flags, and indicate which of the following teletext lines are part of a particular "page" or message. The text lines themselves include a somewhat lesser number of flags and text data. Typically, forty ASCII-encoded bytes are sent per text line, and up to twenty lines can be displayed on the user's screen at once.
FIG. 7 shows in some additional detail the make-up of line 3. It begins with the seventy-eight symbols of HBI data indicated at 72, followed with a bit which is not used, and a number of message bits, each of which is immediately followed by a parity bit. The message bits shown in line 3 of FIG. 7 are each repeated three times and are each protected by parity bits, such that of some 378 total bits, only sixty-two bits of useful data are provided. This data comprises the "system data" used by the subscription television system of the invention to keep control of a wide variety of system functions. Three different versions of line 3 are required to transmit all the system data needed, and each is transmitted in five successive fields, such that the total system data transmission consumes fifteen total field transmissions. A sixteenth field is not used for transmission of system data. The fact that the system data transmitted in line 3 includes a service key which is changed every 16 frames, i.e., on the order of three times per second. This service key must, of course, be accurately received for the decoder to work properly. Therefore, it is transmitted redundantly and in combination with extensive parity-based error correction to ensure correct reception of the service key, as well as the other system data.
The key contained in line 3 is also used to unscramble the location of the color burst signal occurring during the HBI, which varies from the exemplary position shown in FIG. 4.
Teletext is transmitted in a bipartite format. Teletext is transmitted in the form of a number of text lines or rows, making up a page of text. The rows making up the page are preceded in transmission by a teletext header. The header indicates the fact that a teletext page follows and indicates its page number. A decoder looking for a particular page number, for example, a template page, scans the teletext page numbers provided in the teletext headers for the particular page of interest. When the page number sought is detected, the decoder then selects the following page, that is, selects for storage all the teletext lines which follow until the next teletext header line is identified.
FIGS. 8 and 9 show respectively the formats of the teletext header and text lines. In FIG. 8, the teletext header 90 is shown as comprising a thirty-two bit teletext identifier 92. This field simply indicates that this particular line of the vertical blanking interval is a teletext line, as opposed to, for example, an addressed packet. The next thirty-two bit area 94 contains various control flags, which are discussed in detail below. The teletext header then contains a 128 bit area 94 contains various control flags, which are discussed in detail below. The teletext header then contains a 128 bit field 96 which identifies the number of the page which is comprised by the following text lines. The page number is a sixteen bit number, each bit of which is encoded as a eight bit byte. The flags 94 are similarly encoded: that is, a flag which is either a "1" or a "0" data value is nevertheless encoded as an eight bit byte for transmission, so as to enable its correct detection more probable than if it were simply a single bit flag. For the same reason, the page number is a 128-bit word in which each eight byte indicates whether the corresponding bit is a 1 or a 0, again for extremely reliable detection of page numbers. Finally, the last 165 bits 98 of the teletext header 90 are not used.
The flags 94 include a header flag 94a which indicates whether the teletext line is a header or is not, a linked page flag 94b indicating whether the subsequent page of teletext is one of a number "linked" or related to the present page, an encrypted page flag 94c indicating whether the subsequent page is encrypted or not, and a box page flag 94d indicating whether the text shown in the subsequent page should be displayed against a video background or a black background.
The significance of the flags is as follows. The header flag 94a simply indicates whether a particular teletext line is a header or is a line of text. The linked page flag 94b is used to signify to the decoder that a subsequent page contains data needed to complete the message begun in the present page. For example, if a teletext message is too long to fit into a single page comprising twenty 40-character lines of text, the user typically will desire to see the subsequent text page. The linked page flag 94b is used to alert the decoder to this fact and to cause it to copy the page of text having the next higher page number into a random access memory, such that if the user then indicates that he wishes to see the subsequent page of text, it already stored in the random access memory. In this way, the entire message can be displayed more or less immediately, as opposed to waiting for a subsequent transmission of succeeding pages, which may not occur for on the order of several minutes in a very busy system. The linked page flag 94b thus provides an opportunity to improve the teletext service to the user. More particularly, any number of pages can be linked to provide lengthy text messages, e.g., stock price quotations or the like, which can efficiently be read in sequence.
The encrypted page flag 94c indicates whether the text found in the subsequent text lines making up a page is encrypted or not. In many cases, of course, there is no reason to encrypt the teletext, for example, the message is not private, or if its loss will not be damaging to the system integrity, as would be, for example, the loss of control over a first-run motion picture or the like. Hence, many teletext lines are not in fact encrypted.
Finally, the box flag page 94d indicates to the decoder that the teletext in a subsequent page is to be superimposed over whatever video is on the screen at the time, instead of being displayed against a plain background. This flag is useful for several purposes. For example, closed-captioned teletext, providing lines of dialogue and the like so that the hearing-impaired can follow the text of a film, is clearly best provided in this way, such that a viewer can simultaneously see the text and the video. On the other hand, important system messages, such as warnings of community dangers and the like, may be more dramatically or effectively presented against a plain background. Hence, this option is provided and is controlled by the box page flag 94d as noted.
FIG. 9 shows the structure of an individual text line 100 up to twenty of which may make up a page of text. As in the case of the teletext header of FIG. 8, the first thirty-two bits 102 of the text line 100 are a teletext identifier. These are identical whether the teletext line is in fact a header or is a text line. The next eight bits are a header flag 103, which is identical to the header flag comprised by flags 94 of the header line 90, that is, it is an eight bite byte indicating that the teletext line is in fact a text line 100 and not a teletext header 90. The following 320 bits are devoted to the transmission of forty bytes of textual data. Typically, these are encoded according to the usual ASCII standards, whereby each byte is seven bits of data plus a parity bit for error detection. Thus, each text line transmits forty characters which may be any alpha-numeric character found in the ASCII character set. The last seventeen bits 108 are not used.
Thus, in practice, the broadcast transmitter transmits a sequence of teletext lines in lines 9-13 of the vertical blanking interval (see FIG. 6). Up to twenty textlines 100 may follow each teletext header 90. The teletext head 90 contains a page number 96 which identifies the following text lines as, for example, belonging to a template useful in displaying billing status, or as including, for example, information concerning the current movie being run, that is, describing its title, its lead characters, it length, and the price the subscriber will be charged for viewing it, or the like. It will be appreciated, therefore, that the teletext lines in any given vertical blanking interval may be all text lines 100, since only five teletext lines can be transmitted in a vertical blanking interval. (It will be appreciated by those skilled in the art that this numerical limitation relates to a 525-line NTSC-type signal; the actual numbers of the lines-in the VBI are different in the PAL type 625-line system.)
A 9600 k baud asynchronous data channel for use, for example, by a personal computer is also transmitted over the MAC signal. Additionally, one or more audio channels may be used for data transmission.
Thus, the MAC signal includes video and up to six audio channels, as well as text and data. Typically, a MAC decoder includes one pair of audio outputs. These outputs are generally dedicated for stereo audio output to accompany the video for a transmitted program. However, since up to six channels of audio output are available, the other four channels may, for example, carry a second language to accompany the video for the transmitted program, radio transmissions, or high speed reassigned data. The availability of these additional audio channels, along with text and data, provides flexibility to system operators. Thus, in prior systems, a subscriber could, for example, listen to a radio station transmitted over transponder channel 4. A transponder is a microwave repeater which receives, amplifies, downconverts and retransmits signals at a communication satellite. To listen to the radio station, the subscriber tuned to channel 4 and actuated a key designated "RADIO" on his or her handheld remote or on a keypad on a front panel of the decoder. A radio menu offering one or more selections would then appear on the screen to invite the subscriber to make a selection, thereby enabling the subscriber to listen to the selected radio station. A text screen identifying the radio station tuned was then displayed. While such an arrangement utilizes the features of a MAC signal, the above-described procedure can be confusing to a subscriber since the video of transponder channel 4 and the audio associated with the radio station, even though transmitted over the same transponder channel, are typically unrelated. Thus, when a subscriber tunes to channel 4 prior to activating the "RADIO" key, video and audio unrelated to the desired radio station are presented. Further, the subscriber must first consult a program guide to find the appropriate transponder channel and then either again refer to the program guide or to a menu and possibly submenus to listen to the radio station.
Other specific details of a prior art conditional access television system may be found in commonly assigned U.S. Pat. No. 4,890,319, incorporated herein by reference.
As noted, the MAC signal may also be utilized to transmit text for display on a subscriber's television. Text screens may, for example, provide weather reports, sports updates, and stock market quotations. Typically, such information is presented on several screens through which a subscriber may page by using, for example, a "NEXT" key. However, if the information is presented on a large number of different screens, a subscriber will need to page through a number of screens to obtain the information he or she is seeking, resulting in delay and frustration.
Thus, although prior systems have utilized the inherent features of a MAC system, present interfaces between the system and a subscriber desiring to use these features can lead to confusion and delay.